Prior to the present invention, the calibration of personal air samplers involved in large part empirically obtained readings and sometimes unreliable manipulation by the user. Many of the calibrators employed bubble flowmeters.
An early patent to Gussman et al, U.S. Pat. No. 3,994,153, was directed to the calibration of the rotameter used to measure flow.
Conkle et al, in U.S. Pat. No. 4,569,235, maintain substantially constant air flow in an air sampler by monitoring flow rate change and using a signal representative of the flow rate change to manipulate the pump.
Padden et al, in U.S. Pat. Nos. 5,456,107 and 5,440,925, describe generating electrical signals representing flow rates and as a function of known volumes in one or more enclosures traversed by a piston. It is suggested that these signals may be used in data logging and report generation, but the device is not tied directly to the sampler pump for calibration in the manner of the present applicants.
Flow rates are calculated from the velocity of air in a sampler, as described by Buchan in U.S. Pat. No. 4,375,667. The flow rates are then integrated within the instrument over a sample period to provide an indication of the volume sampled, which the authors say enables them to obviate a calibration step. This disclosure employs a microprocessor and converters for processing the data obtained, but still may be said to calibrate only for a current sample and not as a standard for use over an extended period of time.
Ogden et al, in U.S. Pat. No. 5,551,311, uses a personal computer and describes a calibrating apparatus connected to it from the sampler. The system, however, does not utilize the data in the manner of applicants. See also Ogden et al U.S. Pat. No. 5,646,357.
Peck et al, in U.S. Pat. No. 5,107,713, describes a procedure which is manually repeated for several flow measurement readings; the user is prompted to enter the readings on a CPU, which further manipulates them based on an empirical compilation to establish a relationship between air flow, pulse width modulation, and pump motor RPM, using also a flow calibration meter. As described, the device is essentially self-calibrating, but does not tie the data generation to the calibrating device as applicants do.
The prior art approaches to calibration are not conducive to the calibration of a large number of personal air samplers within a short period of time. One may, for example, manually change the speed of the pump and then read a calibrator such as that described by Ogden et al above, marking down the data as it is collected. This process is susceptible to errors in setting the pump, recording the setting of the pump, and recording the reading of the calibrator. Over a period of time and a series of calibrations, errors are statistically likely.